Conventionally, electric vehicles, hybrid vehicles and fuel cell vehicles that run using electric motor as a driving source have been known. In such a vehicle, a battery is mounted for supplying electric power as the energy to the electric motor.
A lithium ion secondary battery that can achieve high energy density and high output density has been developed and used as a large-capacity power source for a hybrid vehicle or an electric vehicle. When the lithium ion secondary battery is used in a hybrid vehicle or an electric vehicle, a plurality of unit batteries (battery cells) are connected in series, to ensure high output.
When the batteries are connected by means of a connecting member, however, battery output decreases because of electric resistance of the connecting member. Further, the larger the volume occupied by the connecting member with respect to the volume of batteries as a whole, the lower the output density or energy density of the batteries.
A bipolar battery is one of the batteries that can be a solution to such a problem, and it enables reduction in resistance between battery cells and reduction in size. A bipolar battery generally has a structure in which a plurality of bipolar electrodes are stacked with an electrolyte interposed. Here, a bipolar electrode means an electrode having a collector member formed as a sheet, with positive electrode active material provided on one surface and negative electrode active material provided on the other surface, of the collector member. The bipolar secondary battery may have various shapes, and as an example, it has a thin plate shape.
Japanese Patent Laying-Open No. 2004-158306 discloses, as an example of such a bipolar battery, a solid polymer battery in which positive and negative electrodes containing polymer electrolyte are provided on opposite sides of a solid polymer electrolyte layer. The solid polymer battery is characterized in that, at portions close to the collector of positive and/or negative electrode, the polymer electrolyte is gelatinized, and at portions closer to the solid polymer electrolyte of positive and/or negative electrode, the polymer electrolyte is fully solidified. Japanese Patent Laying-Open No. 2004-158306 discloses that the battery is housed below the seat at the central portion of a vehicle, in an engine room or inside the roof.
At the time of a car accident, for example, when the vehicle body is hard-hit, various electric circuits operating with the electric power supplied from the battery may possibly fail. If electric power should be supplied from the battery to a broken electric circuit, excessive current would flow through the broken electric circuit, possibly generating heat or smoke from the circuit.
It is often the case that a vehicle has a controller that suspends or limits charging/discharging of battery when abnormality is detected. Namely, in case of vehicle malfunction or accident, an output path of power supply circuit including the secondary battery is automatically shut off. At the time of an accident, however, the controller itself could be broken.
Therefore, it is preferred that the vehicle has a device (or a method) that can shut off the output path of power supply circuit including the secondary battery at the time of vehicle malfunction or accident, in addition to the controller such as described above. Japanese Patent Laying-Open No. 2004-158306, however, does not disclose any method of shutting off the output path of secondary battery at the time of vehicle malfunction or accident.